Implement non-player entities

[Azuelle]

The foundations have already been laid in 71fea26.
Entities have a base class Entity which derives into Bomb, PowerUp, Traps, etc. (add more when new entity types are needed, maybe barricade?), each can be assigned to different persons if needed.
Each entity instance has a timer and a onDeath() function (differs from type to type) which determines when it "dies" and what it does then, e. g. Bombs explode and kill other entity in range.

Current Progress

• Base type Entity
• Bomb
• Barricade
• PowerUp
• Trap